1. Field of the Invention
This invention relates generally to power supply pulse width modulation (PWM) control systems and methods, and more particularly to a power supply PWM method and system that uses peak current program mode (CPM) control for large duty ratios with a smooth transition to voltage mode control at small duty ratios down to zero duty ratio.
2. Description of the Prior Art
Current program mode (CPM) is a popular PWM control technique used in switched mode power supply technology. The duty ratio is determined when the switch current reaches a peak value that is requested by a control voltage. Often, the control voltage is a compensated version of the error between a power supply output voltage and a fixed reference. At normal operating levels, the switch current and the requested peak current are larger than the noise in the system, and they are sufficiently centered between comparator supply voltages and error-amplifier supply voltages in order to have smooth operation at large duty ratios and large currents. When a small duty ratio is required however, (i.e., below a few percent), the current sense signal becomes too small and too close to ground for the PWM comparator to successfully distinguish the greater of the current sense signal and the control voltage signal. This scenario is encountered in a PWM controlled power supply operating at light load conditions. The unfortunate result is that the duty ratio erratically jumps between zero and a minimum duty ratio. The result to the rest of the converter is that the output voltage will lose regulation and transformer isolated topologies may experience over-current conditions from saturation that is caused by the erratic behavior. Small duty ratios and high switching frequencies are also limited by the delay in the logic that is required to detect and latch the pulse width information each switch cycle.
In view of the foregoing,, a need exists for a power supply pulse width modulation (PWM) scheme that improves light load/low duty ratio performance in current program mode (CPM) controlled switched mode converters and that increases speed by eliminating the PWM latch and its associated delay.